1. Field of the Invention
The present invention generally relates to an apparatus and method for detecting blockage of impulse lines. More specifically, the present invention relates to an apparatus and method for detecting blockage of impulse lines that are coupled to a differential pressure transmitter, wherein the differential pressure transmitter is configured to measure the differential pressure of a fluid that flows through a tube.
Priority is claimed on Japanese Patent Application No. 2006-84524, filed Mar. 27, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
It is well-known to a person skilled in the art to which the invention pertains that a differential pressure transmitter can be used to detect the differential pressure between upstream and downstream of an orifice that is provided in a tube. The differential pressure transmitter is coupled to upstream and downstream impulse lines. These upstream and downstream impulse lines are further coupled to the tube at upstream and downstream positions thereof, respectively. The upstream and downstream positions are positioned upstream and downstream of the orifice, respectively. The fluid may have different pressures upstream and downstream of the orifice. The pressure of a fluid upstream of the orifice will hereinafter be referred to as an upstream pressure. The pressure of a fluid downstream of the orifice will hereinafter be referred to as a downstream pressure. In general, the upstream pressure is higher than the downstream pressure. Thus, the upstream and downstream impulse lines may also be referred to as higher and lower pressure impulse lines, respectively.
The upstream and downstream impulse lines are respectively configured to allow the upstream and downstream pressures to travel from the tube to the differential pressure transmitter. The differential pressure transmitter is configured to measure the traveled upstream and downstream pressures so as to detect the differential pressure based on the measured upstream and downstream pressures.
A blockage of the impulse line or impulse lines can prevent the differential pressure transmitter from accurately detecting the differential pressure. In view of monitoring the fluid in the tube, it is important to detect or diagnose that a blockage is present or absent in the impulse line or impulse lines.
A conventional method of detecting a blockage in the impulse line or impulse lines will be described. It is assumed that Dps(i) represents data sets that are related to differential pressure in the normal state in which the impulse lines are free from any blockage. These differential pressure data sets are obtainable in time series from the differential pressure transmitter. The differential pressure has fluctuations Fdps(i) that are given by the following equation (1), where Dps(i) is the current differential pressure data set that is currently obtained from the differential pressure transmitter, and Dps(i−1) is the last differential pressure data set that was last obtained from the differential pressure transmitter. The differential pressure fluctuations Fdps(i) have a variance Vas (root mean square) that is given by the following equation (2), where N is the total sample number of the differential pressure data sets Dps. The variance Vas that is previously found before the diagnosis is made will hereinafter be referred to as “standard fluctuation variance”.Fdps(i)=Dps(i)−Dps(i−1)  (1)Vas=Σ{Fdps(i)2}/N  (2)
The differential pressure fluctuation Fdps(i) and the fluctuation variance Va are determined based on the differential pressure data sets Dps(i) every time the impulse lines are diagnosed during the actual operation of the plant. The above-mentioned equations (1) and (2) can be used to determine the differential pressure fluctuations Fdps(i) and the fluctuation variance Va, respectively.
The square root (D′(Va/Vas)) of a ratio of the fluctuation variance Va to the standard fluctuation variance Vas will be introduced. This value D′(Va/Vas) can be calculated, where the standard fluctuation variance Vas is previously obtained before the diagnosis is made, and the fluctuation variance Va is obtained at the time of diagnosing the impulse lines. The fluctuation variance Va depends on a blockage of the impulse line or impulse lines. This means that the value D′(Va/Vas) also depends on a blockage of the impulse line or impulse lines. Thus, a blockage of the impulse line or impulse lines can be detected by detecting the value D′(Va/Vas).
For example, if both the higher and lower pressure impulse lines have a blockage, then the fluctuation variance Va of the differential pressure becomes smaller than the standard fluctuation variance Vas and also the value D′(Va/Vas) becomes smaller than 1. If either the higher or lower pressure impulse line has a blockage, then the fluctuation variance Va becomes larger than the standard fluctuation variance Vas and also the value D′(Va/Vas) becomes larger than 1. If both the higher and lower pressure impulse lines are free of any blockage, then the fluctuation variance Va approaches the standard fluctuation variance Vas and the value D′(Va/Vas) approaches 1. By comparing the value D′(Va/Vas) to the predetermined threshold, for example, 1, it can be determined whether both or either one of the higher and lower pressure impulse lines have a blockage or the both lines are free of any blockage.
Instead of the above-mentioned equation (1), the following equation (3) can be used to find the differential pressure fluctuation. In case of using the above-described equation (1), the calculated differential pressure fluctuation may reflect a transitional variation component of the differential pressure. The transitional variation may be rising and dropping of the differential pressure.Fdps(i)=Dps(i)−2Dps(i−1)+Dps(i−2)  (3)
However, in case of using the last-mentioned equation (3), the calculated differential pressure fluctuation is free from the transitional variation component of the differential pressure.
Meanwhile, the variance of the differential pressure fluctuations may vary depending on not only blockage of the impulse lines but also the flow rate of a fluid in the tube. The above-described standard fluctuation variance Vas is an experimental value that is measured at a predetermined or fixed flow rate of the fluid in the tube. During the actual operation of the plant, variation in the flow rate of the fluid may cause variation of the above value D′(Va/Vas). However, this variation is independent of the blockage rate of the impulse line or impulse lines. This means that the above-described threshold and the standard fluctuation variance should be set by taking into account the flow rate of the fluid. In other words, to avoid a blockage misdiagnosis it is necessary to update the above-described threshold and the standard fluctuation variance based on variation in the flow rate of the fluid.
There has been developed another conventional technique to correctly detect or diagnose a blockage of the impulse line or impulse lines. This detection or diagnosis is made independently of the flow rate of the fluid. The higher static pressure fluctuations Fsph(i) are calculated in accordance with the following equation (4) by using data sets Sph(i) related to the higher static pressure of a fluid. The higher static pressure is the static pressure of a fluid upstream of the orifice. The higher static pressure data sets Sph(i) are obtainable in time series from the differential pressure transmitter. Also, the lower static pressure fluctuations Fspl(i) are calculated in accordance with the following equation (5) by using other data sets Spl(i) related to the lower static pressure. The lower static pressure is the static pressure of a fluid downstream of the orifice. The lower static pressure data sets Spl(i) are also obtainable in time series from the differential pressure transmitter.Fsph(i)=Sph(i)−Sph(i−1)  (4)Fspl(i)=Spl(i)−Spl(i−1)  (5)
Subsequently, the sum of squares (Ssph) of the higher static pressure fluctuations Fsph(i) is calculated in accordance with the following equation (6). The other sum of squares (Sspl) of the lower static pressure fluctuations Fspl(i) is calculated in accordance with the following equation (7).Ssph=Σ{Fsph(i)2}  (6)Sspl=Σ{Fspl(i)2}  (7)
There is hereby introduced a ratio (D=Ssph/Sspl) of the sum of squares (Ssph) of the higher static pressure fluctuations Fsph(i) to the sum of squares (Sspl) of the lower static pressure fluctuations Fspl(i). This ratio (D=Ssph/Sspl) depends on a blockage of the impulse line or impulse lines. Thus, a blockage of the impulse line or impulse lines can be detected by detecting the ratio (D=Ssph/Sspl).
If the higher pressure impulse line is completely blocked, then the calculated value (Ssph) shall theoretically be equal to zero, and the other calculated value (Sspl) shall theoretically be a predetermined threshold as a non-zero value. Thus, the ratio (D=Ssph/Sspl) shall also be equal to zero. Actually, however, the differential pressure transmitter generates a noise-containing output signal. The noise of the output signal may cause the ratio (D=Ssph/Sspl) to be a non-zero value, for example, approximately 0.05.
If the lower pressure impulse line is completely blocked, then the calculated value (Sspl) should theoretically be equal to zero, and the other calculated value (Ssph) shall theoretically be a predetermined threshold as a non-zero value. Thus, the ratio (D=Ssph/Sspl) shall theoretically be infinite. Actually, however, the output signal noise may cause the ratio (D=Ssph/Sspl) to be a non-infinite value, for example, approximately 20. If both the higher and lower pressure impulse lines are free of any blockage, then both the calculated values (Ssph) and (Sspl) are close to each other. Thus, the ratio (D=Ssph/Sspl) shall be close to 1. Accordingly, by detecting the ratio (D=Ssph/Sspl), it can be determined which impulse line is blocked or both the impulse lines are not blocked.
If the flow rate of a fluid in the tube is increased, then both the calculated values (Ssph) and (Sspl) are also increased together. Also, if the flow rate is decreased, then both the calculated values (Ssph) and (Sspl) are also decreased together. Thus, the ratio (D=Ssph/Sspl) is independent of the flow rate. It is possible to accurately detect or diagnose a blockage of the impulse line or impulse lines independently of the flow rate of the fluid in the tube. The higher and lower static pressure fluctuations Fsph(i) and Fspl(i) can be determined in the same manner as described above.
The above-described and other technical backgrounds are disclosed in Japanese Unexamined Patent Applications, First Publications, No. 2004-132817, No. 2004-294175, and No. 2005-274501.
As described above, the second conventional technique can be used to accurately detect or diagnose a blockage of the impulse line or impulse lines independently of the flow rate of a fluid in a tube. The second conventional technique makes it difficult to set a threshold that is compared to the ratio (D=Ssph/Sspl).
If the higher pressure impulse line is completely blocked, the actual ratio (D=Ssph/Sspl) is approximately 0.05. If the lower pressure impulse line is completely blocked, the actual ratio (D=Ssph/Sspl) is approximately 20. If both the higher pressure impulse lines are not blocked, then the actual ratio (D=Ssph/Sspl) is nearly equal to 1. Thus, the threshold can be set 1. In this case, if the actual ratio (D=Ssph/Sspl) is less than 1, then it is determined that the higher pressure impulse line is blocked while the lower pressure impulse line is not blocked. If the actual ratio (D=Ssph/Sspl) is more than 1, then it is determined that the lower pressure impulse line is blocked while the higher pressure impulse line is not blocked.
In other words, if the higher pressure impulse line is blocked, then the actual ratio (D=Ssph/Sspl) is in a wider range of 1<D≦20. If the lower pressure impulse line is blocked, then the actual ratio (D=Ssph/Sspl) is in a narrower range of 0.05≦D<1. The determination range indicating that the higher impulse line is blocked is different in width from the other determination range indicating that the lower impulse line is blocked. This makes it inconvenient and difficult for a user to set the threshold.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved apparatus and/or method. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.